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Issue 15, 2003
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Overcrowded 1,8-diazafluorenylidene-chalcoxanthenes. Introducing nitrogens at the fjord regions of bistricyclic aromatic enes

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Abstract

The effects of introducing nitrogen atoms in the fjord regions and chalcogen bridges on the conformations of overcrowded bistricyclic aromatic enes (1, X ≠ Y) (BAEs) were studied. 9-(9′H-1′,8′-Diazafluoren-9′-ylidene)-9H-thioxanthene (12), 9-(9H-1′, 8′-diazafluoren-9′-ylidene)-9H-selenoxanthene (13), 9-(9′H-1′,8′-diazafluoren-9′-ylidene)-9H-telluroxanthene (14), 9-(9′H-1′,8′-fluoren-9-ylidene)-9H-xanthene (15) and 9-(9′H-1′,8′-fluoren-9′-ylidene)-9H-fluorene (16) were synthesized by two-fold extrusion coupling reactions of 1,8-diaza-9H-fluoren-9-one (19)/chalcoxanthenthiones (24–27) (or /9H-fluorene-9-thione (30)). The 1′,8′-diazafluoren-9-ylidene-chalcoxanthenes (11) were compared with the respective fluoren-9-ylidene-chalcoxanthenes (10). The structures of 12–16 were established by 1H, 13C, 77Se, and 125Te NMR spectroscopies. The crystal and molecular structures of 12–14 were determined by X-ray analysis. The yellow molecules of 12–14 adopted mono-folded conformations with folding dihedrals in the chalocoxanthylidene moieties of 62.7° (12), 62.4° (13) and 59.9° (14). The folding dihedrals in the respective 1′,8′-diazafluorenylidene moieties were very small, ca. 2°, compared with 10.2/8.0° in (9′H-fluoren-9′-ylidene)-9H-selenoxanthene (7). A 5° pure twist of C9[double bond, length as m-dash]C9′ in 14 is noted. The degrees of overcrowding in the fjord regions of 12–14 (intramolecular non-bonding distances) were relatively small. The degrees of pyramidalization of C9 and C9′ were 17.0/3.0° (12), 17.4/2.4° (13) and 2.2/2.2° (14). These high values in 12 and 13 stem from the resistance of the 1,8-diazafluorenylidene moiety to fold and from the limits in the degrees of folding of the thioxanthylidene and selenoxanthylidene moieties (due to shorter S10–C4a/S10–C10a and Se10–C4a/Se10–C10a bonds, as compared with the respective Te–C bonds in 14). The molecules of 15 and 16 adopt twisted conformations, a conclusion drawn from the 1H NMR chemical shifts of the fjord regions protons (H1 and H8) at 8.70 (15) and 9.00 ppm (16) and from their colors and UV/VIS spectra: 15 is purple (λmax = 521 nm) and 16 is orange–red. A comparison of the NMR spectra of 11 and 10δ = δ(11) – δ(10)) showed substantial downfield shifts of 0.56–0.62 ppm of the fjord regions protons of twisted 15 and 16: Δδ (C9) were negative (upfield): −4.0 (12), −3.7 (13), −3.4 (14), −7.1 (15), −5.0 ppm (16), while Δδ (C9′) were positive (downfield) = +6.8 (12), +6.5 (13), +5.8 (14), +11.7 (15), +7.7 ppm (16). In 15, Δδ (C9) – Δδ (C9′) = +18.8 ppm, attributed to a push–pull character and significant contributions of zwitterionic structures in the twisted conformation. The 77Se and 125Te NMR signals of 13 and 14 were shifted upfield relative to the respective fluorenylidene-chalcoxanthene derivatives: Δδ77Se = 17.2 ppm and Δδ125Te = 22.0 ppm. The presence of the nitrogen atoms (N1′ and N8′) in 13 and 14 causes shielding of the selenium and tellurium nuclei.

Graphical abstract: Overcrowded 1,8-diazafluorenylidene-chalcoxanthenes. Introducing nitrogens at the fjord regions of bistricyclic aromatic enes

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Publication details

The article was received on 20 Mar 2003, accepted on 11 Jun 2003 and first published on 30 Jun 2003


Article type: Paper
DOI: 10.1039/B303041E
Org. Biomol. Chem., 2003,1, 2755-2763

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    Overcrowded 1,8-diazafluorenylidene-chalcoxanthenes. Introducing nitrogens at the fjord regions of bistricyclic aromatic enes

    A. Levy, S. Cohen and I. Agranat, Org. Biomol. Chem., 2003, 1, 2755
    DOI: 10.1039/B303041E

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